Supply chain, certification, and substitution of hazardous substances

The purpose of this paper is to provide information on the challenges of substitution of hazardous chemicals linked to Aerospace & Defence value chain complexity and strict performance requirements for certification and qualification.

The aerospace & defence (A&D) sector comprises many different actors operating across a global industrial network. This includes Original Equipment Manufacturers (OEMs) and suppliers, who design, manufacture, process and assemble parts and systems for A&D products. It also includes e.g. companies supplying parts, processing/treating parts, carrying out maintenance, repair and overhaul, as well as companies designing and building final products (e.g. aircraft, launchers, military equipment or their parts). 

Due to the complexity of the products, each OEM will deal with hundreds of suppliers for each product/platform type. A&D products are required to operate safely in extremes conditions and for decades due to their very long service lives. In addition, there are strict technical, performance, high reliability, and safety requirements for A&D products, their components and the necessary MRO (maintenance, repair and overhaul) over service life.  As such, substitution of substances can pose several technical and logistical challenges. Each A&D company that designs and integrates the final product (e.g., aircraft, engines, radar, rocket engines, and other defence systems), is responsible for their own product qualification, validation, and certification, according to airworthiness regulations or defence/space customer requirements. 

Sector certification / qualification process 

Within the A&D Sector, the Design Owners provide technical specifications, according to which aircraft, spacecraft, or military equipment is produced. In the EU, any aircraft is certified for airworthiness by the European Aviation Safety Agency (EASA), while military equipment follows a similar certification process that is governed by respective national Ministries of Defence. Space products, in turn, must meet strict safety requirements for launch sites.

After certification, the specifications of an aircraft, space, or defence product are fixed; production and maintenance and repairs (MRO) can be performed only in compliance with these specifications (“repair-as-produced-principle”).  This means, that the reliance on certain substances is ‘locked in’ and substitutions can only be made with extensive testing to show that any alternatives perform reliable and equivalent to the incumbent solutions. In many cases, re-certification of affected products will be required when substituting.

Usually, this reliance on certain substances in A&D products is due to their presence in formulations or materials required for the design, rather than certain substances being directly included. For example, the use of a particular adhesive or coating or substrate is ‘locked in’ to the design certification. Each individual adhesive, coating or substrate etc. is specified because they have been demonstrated to fulfil the strict technical and performance requirements for each part of the design where they are used.
Civil aircrafts are subject to the airworthiness requirements which are governed by ‘critical to flight safety’ principle, while defence and space equipment is subject to a principle of ‘mission and launch site readiness’ that dictates the key performance requirements. 

Where specific substances are necessary for A&D products (e.g. because they are present in formulations or materials) their use is critical to flight safety and air- or space-worthiness. The presence of a substance is related to the performance requirements of the formulations, materials or processes containing the substance, which are used to manufacture and assemble parts, and the correct performance of these parts is essential to ensure the performance and product safety of assemblies, sub-systems, systems and ultimately the aircraft into which they are integrated.

It is important to stress that alternatives are not necessarily a one-to-one solution, as very often the use of a certain alternative on one part does not mean the same alternative can be used on other parts, without further extensive qualification testing and certification. This is also due to the different locations and environments of parts within a product and the different operational conditions (e.g. temperature, pressure, corrosion, space vacuum, radiation, etc.) that the different parts are subject to. In some cases, multiple different alternatives must be pursued to replace a single out-going solution e.g. where a single alternative does not fully replicate all requirements for all affected parts. In addition, each OEM must qualify and certify an alternative for use throughout all their affected products and processes. 

The certification process for civil aviation involves several stakeholders, as shown below: 

For military equipment ‘mission readiness’ means that at any given time, that equipment is ready to perform according to its intended design, and its performance is not compromised by degradation resulting from e.g., corrosion of parts. Unreliable equipment could result in a myriad of consequences including putting service personnel’s lives at risk.

Requirements for military equipment are set by the individual Ministries of Defence; these requirements (often classified) are stricter than for civil equipment (i.e. often they set a higher performance level), and each country usually sets its own rules.

‘Flight readiness’ for space equipment, like satellites and launchers, refers to the condition and preparation of these systems before they are deemed ready for launch. It’s a comprehensive assessment that ensures all components are fully functional, safe, and capable of performing as intended in the harsh environment of space.

For satellites, this includes confirming that all subsystems (like communication, power, propulsion, and thermal control) are operational and that the satellite is securely integrated into the launch vehicle. For launchers (rockets), it means verifying that the vehicle's propulsion systems, navigation, and payload integration are all in good condition and capable of delivering the satellite or payload to its destination in space.

Flight readiness usually involves rigorous testing, inspections, simulations, and reviews to identify any potential issues that could interfere with the mission. If everything passes, the equipment is considered "flight-ready," meaning it's good to go for launch.

Substitution

Any actor within the supply chain that pursues substitution needs to demonstrate, via extensive testing, that a potential alternative performs at least equally or with equivalent reliability on all types of components where the original formulation/process is used. They need to demonstrate (by law) that by substituting a specific substance the overall performance of an aircraft, spacecraft, or military equipment is not negatively impacted. This can often encompass hundreds of different interlinked components separately.

Any modifications must demonstrate adherence to EASA or launch site requirements for overall air- and space-worthiness respectively, to obtain certification or approval from clients (Ministries of Defence in case of military equipment), meaning that each component must meet the performance and safety standards achieved by the substance / technology currently in use.

Generally, substitution will encompass several phases, as shown below; this is a strict ‘gated process’ (i.e. a next stage of the process can only start after meeting specific targets and/or after a management decision), meaning that the potential for performing the stages in parallel is limited.

In order to identify alternatives, OEMs often rely on the expertise of chemical formulators and on their willingness to invest in research for alternatives. Once formulators provide proposed candidate alternatives against the requirements in each affected OEM specification, the OEMs perform the required tests to verify the initial feasibility. Often, while in the initial phases performance results might show promise, the tests eventually fail due to the high-performance requirements, and the overall process of qualification of possible alternatives might take several years. In some cases, the small volumes needed, and the demanding performance requirements discourage formulators from investing in research aimed at finding alternatives specifically for A&D uses. 

Maintenance manuals specify requirements to ensure performance criteria for certified equipment are maintained when carrying out maintenance activities. Adherence to these manuals is obligatory, as EU law mandates the use of approved data and/or manuals in a maintenance facility (EU No. 1321/2014 Annex II 145.A.45(a)). 

There are other aspects to keep in mind when considering substitution in A&D. For instance, there is a close relationship between the different parts within these complex products, and when substituting a substance or material in one part or process, it can be necessary to assess whether this will have repercussions on the performance of other parts and in turn on the overall performance of the final product (e.g. aircraft; military systems; etc.). 

It should also be noted that in the defence sector the process of substitution is tied to government funding decisions, which may affect the timelines. 

In summary, when substitution is required, it is important to ensure that all aspects of each specific use are properly considered and addressed in the required testing and to recognise that developing and introducing alternatives in A&D is an iterative and lengthy process, where success is not guaranteed and multiple workstreams may need to be undertaken in parallel.